Field of the Invention
The present invention relates to a manufacturing method, a control device, and a manufacturing apparatus of an optical fiber.
Description of Related Art
In general, in order to manufacture an optical fiber, a fiber drawing method of drawing an optical fiber vertically downward from an optical fiber preform along a straight path is employed.
For example, in order to manufacture an optical fiber 5 using a manufacturing apparatus shown in FIG. 5, an optical fiber preform 2 is melted by a heating furnace 11 of a drawing unit 10 and a bare optical fiber 3 is drawn vertically downward therefrom through drawing. The bare optical fiber 3 is cooled by a cooling unit 120 and is thereafter provided with a coating layer by a coating unit 30, thereby obtaining an optical fiber intermediate body 4. The coating layer of the optical fiber intermediate body 4 is cured by a curing unit 40, thereby obtaining an optical fiber 5. The optical fiber 5 is wound by a winder 100 via a pulley 80 and a take-up unit 90.
Regarding a manufacturing method thereof, as a factor that affects productivity, there is a restriction on the height of the entire system. The height of the system is the main factor in the restriction of productivity because there is a need to ensure a sufficient distance for cooling the bare optical fiber obtained by drawing of the optical fiber preform.
When a new facility including a building is established, such restriction can be relaxed. However, this requires an enormous cost. When the enhancement of productivity is further required in the future, there is need to establish a new facility at a higher cost.
As a method of relaxing such restriction, there is a method of using a direction conversion tool having a non-contact holding mechanism.
The non-contact holding mechanism is a mechanism for holding an object under the pressure of a fluid such as air in a non-contact manner, and a direction changing device having the mechanism can change the direction of the bare optical fiber without coming into contact with the bare optical fiber (bare fiber).
When the direction changing device is used, the direction of the bare optical fiber drawn from the optical fiber preform along a first path can be changed to follow a second path that is different from the first path (for example, refer to Japanese Patent No. 5571958 and Japanese Unexamined Patent Application, First Publication No. S62-003037).
Japanese Patent No. 5571958 discloses a manufacturing method of an optical fiber in which a direction conversion tool that has a groove, into which an optical fiber is introduced, and has an opening formed in the groove is used. In this method, gas introduced into the tool through an inflow port is ejected from the opening such that the optical fiber is changed in direction in a state in which the optical fiber is floated by the pressure of the gas.
A direction changing device described in Japanese Unexamined Patent Application, First Publication No. S62-003037 has a guide groove which guides the bare optical fiber, and gas outlets are formed at the bottom surface and both side surfaces of the guide groove (see Example and FIGS. 3 and 4). In the manufacturing method using the direction changing device, the direction of the optical fiber is changed in a state in which the optical fiber is floated by the pressure of the gas blown from the four outlets.
The floatation amount of the bare optical fiber is determined by the balance between the pressure of the gas blown from the inside of the groove of the non-contact holding mechanism, the outer diameter of the bare optical fiber, and a drawing tension applied to the bare optical fiber. Therefore, when the flow velocity of the gas and the outer diameter and drawing tension of the bare optical fiber are constant, the floatation amount of the bare optical fiber becomes constant, resulting in stable drawing.
However, in an actual manufacturing process, due to a variation in the outer diameter of the optical fiber preform, a variation in the drawing velocity of the bare optical fiber, and a decrease in the remaining length of the optical fiber preform, the floatation amount of the bare optical fiber may be changed.
Particularly, in a process (hereinafter, referred to as an acceleration process) of increasing the drawing velocity from a low velocity (for example, approximately 30 m/min) to a steady velocity (generally 1500 m/min or higher) at which a product is manufactured, the outer diameter and drawing tension of the bare optical fiber are not in a state of being controlled to be constant, and the shape of a narrowed portion (neck-down) of the tip end portion of a preform changes with an increase in the drawing velocity minute by minute. Therefore, the outer diameter and drawing tension of the bare optical fiber change, and the floatation amount thereof changes minute by minute.
When the drawing tension is increased after the outer diameter of the bare optical fiber is decreased, the floatation amount of the bare optical fiber is decreased. Therefore, there is concern that the bare optical fiber may come into contact with the inner surface of the groove of the tool. When the bare optical fiber comes into contact with the tool, the bare optical fiber is damaged, and there is a possibility that the strength thereof may decrease.
On the other hand, when the drawing tension is decreased after the outer diameter of the bare optical fiber is increased, the floatation amount of the bare optical fiber is increased, and the stability of the floating state thereof is decreased. Therefore, there is concern that the bare optical fiber may come into contact with the inner surface of the groove of the tool, the bare optical fiber may be damaged, and the strength thereof may decrease.
In addition, when the floating position of the bare optical fiber is changed in the non-contact holding mechanism, the position of the bare optical fiber introduced into the coating unit provided on the downstream side of the non-contact holding mechanism varies, and there is concern that the thickness of the coating may become uneven.
An upstream side means the upstream side in a drawing direction, and a downstream side means the downstream side in the drawing direction. Therefore, a downstream direction is not a constant direction. After the direction of the bare optical fiber is changed by the non-contact holding mechanism, the changed direction is the downstream direction.
The present invention has been made taking the foregoing circumstances into consideration, and provides a manufacturing method, a control device, and a manufacturing apparatus of an optical fiber which achieve the stabilization of a floating position of a bare optical fiber in a direction changing device even when the outer diameter of an optical fiber varies.